It is known to solder a resistor ring, typically a non-linear resistor ring, such as a varistor, to commutator tangs of an electric motor so that resistor elements are connected between pairs of adjacent commutator segments. These resistor elements serve to discharge inductive energy from the winding coils and thus reduce electrical noise.
However, varistors and indeed some other types of resistor are made by a sintering process and are by their very nature extremely brittle. They are easily damaged if excessive heat is applied and consequently, it is necessary to be extremely careful when soldering.